Our studies show that IFN-lambda/IL-28 sustains the activation of the signal transducers and activators of transcription (STAT)1 and STAT2 for longer than 8 hours. This is in contrast from the activation profile of these same STATs that are activated by type I IFNs. We have shown this in the human 2fTGH fibrosarcoma and HaCat immortalized skin keratinocyte cell lines and in B16 mouse melanoma cell line. In addition, IFN-lambda was superior than IFN-alpha in inducing an antiproliferative effect in HaCat, which paralleled detection of apoptotic cells that were not detected with IFN-alpha treatment. In stark contrast with IFN-alpha, biochemical analysis revealed that maintainance of STAT1 activation by IFN-lambda did not require new protein synthesis. Although both IFNs exert an antiproliferative effect on tumor cells, our study shows how these two cytokines do not activate the Jak/STAT pathway in the same manner. To compare the invivo antitumor effects of IFN-lambda, using a human xenograft tumor model, we have treated immunocompromised RagKO mice with either IFN-beta or IFN-lambda carrying established subcutaneous tumors induced by injecting them with human keratinocyte cells expressing oncogenic Ras. Each cytokine alone inhibited further tumor growth. However, IFN-beta was more efficient in shrinking the size of the established tumors. Our ongoing work is to use both cytokines together and determine if their antitumor effect can be augmented when used in combination. These initial findings highlight the potential therapeutic use of IFN-lambda in the treatment of cancer. Our study differs from other reports as we employed recombinant protein rather than tumor cells genetically engineered to secret IFN-lambda. Now we are expanding this work by studying and comparing IFN-lambda to IFN-alpha responsiveness by first detecting IFN-induced STAT activation in paired primary and several metastatic melanoma cell lines that were derived from the same patient. Thus far primary melanoma tumors from 3 different patients only respond to IFN-alpha but not to IFN-lambda thus suggesting that these tumors may not express IFN-lambda receptors. Surprisingly, while all metastatic tumors respond to IFN-alpha, only some gained responsiveness to IFN-lambda. From this initial study, we conclude with the following observations: (1) metastatic tumors that respond to IFN-lambda must also respond to IFN-alpha/beta; (2) Surface IFN-lambda receptors might be methylated or silenced but can be expressed on metastatic tumors-an observation that we will validate once we have FACS antibodies to measure expression of this receptor and (3)metastatic melatoma tumors are heterogeneous in their response to type I and type III IFNs. Therefore, patients may benefit from including IFN-lambda in their cancer treatment.